The implantation of prosthetic valves using percutaneous techniques is a relatively new field of medicine. While percutaneous implantation has yet to achieve widespread acceptance, there are several companies developing such procedures and products. In this field, it has been found that stented valves are particularly well-suited for percutaneous advancement to a treatment site. Stented valves can be divided basically into two groups: self-expanding stented valves and expandable, i.e., internally-expanded, stented valves, which are most commonly expanded by balloons. Self-expanding stents are usually made from shape memory materials, such as nickel-titanium alloys, or Nitinol, which have a high elastic range. Balloon expandable stents are typically formed of a plastically deformable material having a high radial strength, such as such as stainless steel, platinum, iridium, cobalt-chromium, and the like.
Before delivery to a treatment site, a self-expanding stented valve may be compressed and inserted into a small tube. The stented valve is guided, i.e., pushed or pulled, distally, through the tube to a desired location and then released from the restricting tube or sheath. The stent then expands to its set diameter, or until it is restrained from further expansion by lumen walls. The expansion is caused by the stent's internal elastic radial forces. In some cases an additional force applied by, for example, a balloon, expands the stent to its final diameter.
A balloon expandable stented valve may be crimped from a large set diameter to a small crimped diameter and then moved into a patient's body ducts via an introducer sheath. After the stented valve reaches a desired position in the body, the stented valve is then expanded back to its set diameter by an external force, such as that created by an inflatable dilatation balloon.
Although stented valves may be crimped or compressed to a smaller diameter for delivery purposes, it has been found that some degree of recoil occurs after the crimping or compression process. This is a particularly undesirable feature because the stented valve is typically advanced through an introducer sheath. Furthermore, the size of the introducer sheath is limited by the size of the entry into a patient's blood vessel. Therefore, due to recoil and other factors, when the size of the entry into the patient's blood vessel is small, it can be difficult or impossible to advance the stented valve through the introducer sheath.
Accordingly, an urgent need exists for an improved system for assisting in the delivery of medical devices, such as stented prosthetic valves, into a patient's vasculature. It is desirable that such a system be particularly well-suited for use with prosthetic valves formed with self-expanding or balloon expandable stents. It is also desirable that such a system facilitates the delivery of prosthetic valves through introducer sheaths. The present invention addresses this need.